The present invention relates to a method of manufacturing extruded flat multihole aluminum tubes for heat-exchangers by covering both even surfaces of the extruded flat multihole tubes of Al-Cu or Al-Cu-Mn alloy with Zn.
In general, since the film due to the natural oxidation on the surface of aluminum is hard and dense as in a passive state, the corrosion grows in a form of pitting corrosion. For this reason, the covering of aluminum components with Zn is very effective for giving the cathodic anticorrosion to the components. When using aluminum for the heat-exchanger of cars there can exist a salinity (Cl.sup.--) depending on the environment for use, which triggers the occurrence of pitting corrosion that brings about piercing. As a method of preventing said pitting corrosion, it is known that the surface of aluminum components can be covered with a layer of low-potential Zn to be used as a sacrificial anode for the anticorrosion.
In the case of the aluminum component being a plate or an extruded material made by the mandrel system, such methods as clad rolling and clad extrusion are known for the manufacture of clad material. Also, for an extruded material by the porthole system such as a multihole flat tube, a method of dipping into the molten Zn for covering, a method of flame spraying Zn or Zn alloy onto the tube immediately after the extrusion as shown in Japanese Unexamined Patent Publication No. sho 58-204169, or a method of pressing Zn or Zn alloy against the tube immediately after the extrusion as shown in Japanese Unexamined Patent Publication No. sho 58-157522 ;l are known. In particular, the method of flame spraying Zn onto the surface of a tube immediately after the extrusion is low in installation cost and good in workability, thus the practical utilization has been investigated as the handiest method.
However, for the multihole flat tube made of pure aluminum, for example, A 1050, which is used for the condenser of an air conditioner for cars, Zn should be flame sprayed onto the overall surface, and for this reason, it is necessary to dispose four flame spray guns at the top and bottom and right and left sides of the tube as shown in Japanese Unexamined Patent Publication No. sho 58-204169.
In order to flame spray under such conditions, single hole extrusions should be adopted for the extrusiion of the tube, in other words, only one tube can be extruded from the extruder through the die. Therefore, the productivity is very poor resulting in high cost. Generally in the case of the flat multihole tube of this type, it is common to manufacture two or four tubes simultaneously at one extrusion by providing two or four holes to the die. However, when flame spraying, the extrusion cannot avoid being conducted with one tube because of the necessity to adhere Zn to the overall surface. For this reason, the productivity is reduced by a half or one fourth and high cost is unavoidable.
As a result of extensive investigations in view of this situation, a method of manufacturing extruded flat multihole aluminum tubes for heat-exchangers that are highly corrosive resistant and yet low in cost has been developed according to the invention by combining an extrusion process for two or four tubes with a flame spraying installation and further by restricting the extruding material to Al-Cu or an Al-Cu-Mn alloy.